The long range goal of these studies is to provide a detailed understanding of the mechanism by which macrophages process and present antigen to lymphocytes and how H-2I gene function is expressed at the macrophage level. The strategy will be to focus on protocols designed to define and isolate the antigen recognition phase of lymphocyte activation. Experiments will be designed to test the hypothesis that antigen processing represents the catabolism of antigen by macrophages via a phagosome-lysosome pathway. Monoclonal antibodies and antigen specific T cell lines will be used as probes for macrophage associated antigen, and a novel method for determining which components of an infectious agent (Listeria monocytogenes) are most important for inducing protective immunity will be investigated. This will involve the identification of antigenic determinants selectively expressed on the macrophage plasma membrane following antigen processing.